Brief Description of Project:
As global fossil fuel
resources decline the need for significant research to identify
and enable a viable source is evident. While solar energy looks
promising as a clean abundant, readily available, energy source
for generating electricity, solar-derived fuel from biomass
provides a convenient liquid fuel needed for transportation.
However, the conversion (hydrolysis) of biomass (cellulose) to
glucoses that can be fermented into liquid fuel (e.g., ethanol)
is currently very inefficient (<0.5% net yield of energy). A
major drawback to the systematic development of improved
cellulases (enzymes that catalyze the hydrolysis of cellulose)
is the absence of a clear mechanism of action and understanding
of the kinetic and thermodynamic factors important during the
depolymerization events that release sugar molecules.
Cellobiohydrolase I
(CBH I) is one of the most active cellulases known. This
enzyme hydrolyzes cellulose in a “processive” manner,
moving along a cellulose chain liberating cellobiose
residues. CBH I is a multi-domain enzyme, consisting of
a large catalytic domain containing an active site
tunnel and a small cellulose binding domain (CBD)
joined to one another by a 26 amino acid linker peptide
(Fig. 1). The primary aim of this work is to elucidate
the mechanism of action of CBH I on cellulose. The
hypothesis that the enzyme [the grey object in Fig. 1]
moves along a single cellulose chain in a processive
manner suggests that it will oscillate between an
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Figure 1.
Artist rendition of CBH I. The cellulose binding domain
(CBD) interacts with the cellulose surface to detach
cellulose molecules from the biomass, which are then
shepherded to the catalytic domain where they undergo
hydrolysis. |
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extended and compressed
state, in a caterpillar like motion. Due to the flexibility
of the linker, the CBD can remain bound to one site of the
cellulose while the enzyme hydrolyzes the cellulose chain
within a 4nm range. Once the linker becomes compressed to a
very short distance, it has been hypothesized that the
energy of the linker will be enough to free the cellodextrin
chain from the CBD and allow the enzyme to progress down the
cellulose chain. |
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